In a yarn feeder according to EP-B-0171 516, yarn sensors are provided for contactless yarn detection, e.g. to generate control signals for the yarn winding on drive, which maintains a yarn store of predetermined size on the storage drum essentially independent from yarn consumption. Said yarn sensor comprises a Hall sensor or proximity sensor with magneto resistive or inductive sensor elements. Such yarn sensor can also be used as a yarn breakage sensor. The yarn moves a permanent magnet, which is movably provided in the storage drum. The yarn sensor is responding to the movement of said permanent magnet or its distance. E.g. digital Hall sensors are available at reasonable costs and operate reliably. However, it is a drawback that the respective output signal change occurs, depending from the motion direction of the permanent magnet or a body of magnetic and/or electromagnetic flux conducting material not at the same distance of the permanent magnet from the sensor element, resulting in inaccurate information with respect to the motion and/or the presence or absence of the yarn, respectively. Since the hysteresis, e.g. of digital Hall sensors, is fluctuating within a production series and since in one and the same yarn feeder several yarn sensors can be provided, until now an inaccurate control behaviour due to the fluctuation of the yam sensors had to be taken as given, undesireably leading to yarn stores having too big or too small sizes and to disturbances of the operation of the yarn feeder. This is true for Hall elements as well as for magneto resistive or inductive sensor elements as sensor element.
A magnetically activated proximity switch according to EP-A-0 620 647 is surveying the position and the movement of a magnet coupled to an actuating piston. A control circuit comprises a Hall sensor actuated with sufficient voltage by means of a sensor-pulser to carry out an evaluation of the magnetic field. After an evaluation the voltage for the entire sensor is decreased by the sensor-pulser in order to secure the output signal of the sensor.
A Hall effect sensor as known from GB-A-2 276 246 comprises a Hall element, the output of which is connected with a comparator member actuated by a reference voltage in order to digitize the output signal of the Hall element.
It is an object of the invention to create a yarn feeder of the kind as disclosed in which control signals derived from the response of the yarn sensor exactly are corelated to a certain distance between the permanent magnet or body and the sensor element irrespective whether the permanent magnet or body is just approaching the sensor element or is moving away from the sensor element.